We have proposed a simple cross-bridge model of muscle contraction in which, during each cycle of ATP hydrolysis, the myosin cross-bridge alternates between a conformation in which the cross-bridge binds strongly to actin, and a conformation in which the cross-bridge weakly to actin and is in rapid equilibrium between attached and detached cross-bridge states. In the present study we tested several predictions of this model. First, using 0-18 exchange, we tested whether the two myosin heads act independently in their interaction with actin and ATP as predicted by our model. Second, we tested whether the effect of vanadate is different in vivo and in vitro as has been reported by other workers, and if so, whether this differential effect is consistent with our model. Finally, we used pre-steady-state kinetic studies to determine whether the rate of dissociation of actomyosin by the ATP analogues, AMPPNP and PPi, is similar in vitro and in vivo as predicted by our model. In all of these studies the results were consistent with our model of muscle contraction.